Diphenylmethane dicarbamates are useful precursors for the preparation of diphenylmethane diisocyanates (MDI) without using phosgenes. Their derivative in 4,4'-form, namely, 4,4'-diphenylmethane diisocyanate, which is generally known as pure MDI, is an advantageous starting material for the production of polyurethane elastomers, spandex fibers and artificial leather coatings. There has been a rapid increase in demand for pure MDI. Therefore, there is a need to develop a commercial process for producing diphenylmethane dicarbamates from which the pure MDI can be manufactured.
Conventionally, these diphenylmethane dicarbamates are produced by reacting N-phenylcarbamates with a condensing agent such as formaldehyde, paraformaldehyde, methylal or trioxane in the presence of an acid such as mineral acid or organic sulfonic acid. If relatively severe conditions are used in this reaction, for example, if a strong acid is used in a large amount, the reaction temperature is high or the reaction period is extended, not only is the desired diphenylmethane dicarbamate produced but also polynuclear polymethylene polyphenylcarbamates having the following formula are produced in a significant amount: ##STR1##
(wherein R is an alkyl group, aromatic group or an alicyclic group; z is an integer of 1 or more). Furthermore, if a strong liquid acid is used much difficulty and hence a lot of cost is entailed in separating the acid from the reaction mixture and recovering the same in a reusable form.
In order to eliminate this defect with the recovery of acids, a method was proposed for using an aqueous acid solution having a concentration of 10% or higher (British Pat. No. 2,044,252, Japanese Patent (OPI) Nos. 81850/80 and 81851/80 and Chemical Abstracts 93 169057e). This method is effective for acid recovery because as shown in their working examples, if aqueous acid solutions having a concentration of not more than 50% are used, the acid can fairly easily be separated from the organic phase in the form of layers. However, the presence of a great amount of water renders it difficult to complete the reaction without leaving a significant amount of compounds having a methylene-amino bond(--CH.sub.2 --N&lt;) wherein the methylene group is bonded to the nitrogen atom in the carbamate group. In order to complete the reaction without these compounds, less water must be used to increase the acid concentration to, for example, 80% or higher. However, this causes the hydrolysis of the starting compound or the reaction product, or leaves them to dissolve in the concentrated aqueous acid solution in a large quantity, and as a result, the separation of the product from the acid solution becomes difficult.
In any event, it is not industrially advantageous to carry out a one-step condensation of N-phenylcarbamates with an aqueous solution of acid and to use the resulting product in the preparation of isocyanates. More specifically, dinuclear, trinuclear or other polynuclear compounds having the methylene-amino bond cannot be easily separated from the condensation product containing diphenylmethane dicarbamates and polymethylene polyphenylcarbamates. If the condensation product containing these compounds with the methylene-amino bond is decomposed thermally, these compounds do not provide the desired isocyanates. Furthermore, they enter into various side reactions with the isocyanates derived from the carbamates such as diphenylmethane dicarbamates, and in consequence, the yields of the desired isocyanates are reduced. In addition, the resulting byproducts cannot be easily separated from the desired isocyanates, particularly, the polynuclear polymethylene polyphenyl isocyanates, and they are in all cases present in the final product generally referred to as a polymeric isocyanate, and properties of the product are impaired.
It is therefore necessary to perform the condensation of N-phenylcarbamates in such a manner that a minimum amount of the compounds with the methylene-amino bond is left in the condensation product. One method that has been proposed for attaining this object is described in U.S. Pat. No. 4,146,727, wherein these compounds within the methylene-amino bond are subjected to a rearrangement reaction, under substantially anhydrous conditions, with a protonic acid catalyst having a strength of at least the magnitude of a 75% sulfuric acid, or a Lewis acid at a temperature of 50.degree. to 170.degree. C. so as to rearrange the methylene group, which was bonded to the nitrogen atom, to bond to the benzene ring. However, this method must use a large amount of concentrated sulfuric acid or paratoluenesulfonic acid and again requires complicated procedures and great cost for separating and recovering these acids from the reaction mixture.
Japanese Patent (OPI) No. 7749/81 and Chemical Abstracts, 94 209480s propose a method for producing polymethylene polyphenylcarbamate by heating only bis(N-carboalkoxyanilino)methane, which is a compound having the methylene-amino bond, in the presence of an acid catalyst. However, this method is not ideal for selective production of the diphenylmethane dicarbamate because it causes not only the desired rearrangement reaction but also the undesired condenstion reaction, and trinuclear and other polynuclear polymethylene polyphenylcarbamates are formed as byproducts in addition to the desired diphenylmethane dicarbamate. Furthermore, the reaction is slow and the rearrangement reaction is not completed without leaving the residual bis(N-carboalkoxyanilino)methane in the reaction product.
U.S. Pat. No. 4,319,018, British Pat. No. 2,054,584, Japanese Patent (OPI) No. 12357/81 and Chemical Abstracts, 94 124715t propose a method for producing diphenylmethane dicarbamates and polymethylene polyphenyl carbamates by reacting N-phenylcarbamates with formaldehyde or its precursor in the presence of both an acid catalyst and the compounds having the methylene-amino bond. However, this method is unable to reduce the content of the compounds with the methylene-amino bond, and the compounds are unavoidably left in the condensation product in an amount as much as ten-odd percent by weight.